The template approach

The quickest way to change the view template would be to simply change the text in the template. A safer way - strongly recommended, because it will survive a Foswiki update - is to create your own custom skin. That may sound like an awful lot of work, but in reality a skin may be as much as 1 file that consists of only a few lines of code.

Example
Let's say you want to simplify the bottom toolbar and remove all links except for "More topic actions". You would need to target %TMPL:DEF{"topicactionbuttons"}%, which is located in view.pattern.tmpl.

The steps you would need:

Think up a name for your skin. Let us use myskin for now.

Create a new (empty) view template file in /templates called view.myskin.tmpl.

The topic template approach (using VIEW_TEMPLATE)

Template overrides can be written in a topic, a so-called topic template. Topic template names end with ViewTemplate, for instance BlogPostViewTemplate.

Call this template by setting VIEW_TEMPLATE:

* Set VIEW_TEMPLATE = BlogPostView

Note that you don't use the Template extension now

You can make this setting hidden by writing this in the Settings screen: go to More and then Topic Settings

We can use the same example as above, but extend it to also replace the Edit and Print buttons at the top with a More Actions button. In the topic you want to use as view template, for instance OnlyMoreView, write:

You can also test this by appending ?template=OnlyMoreView to the url.

Reference: pattern skin template INCLUDE tree

The view script invokes the view template: (Note that to simplify customization of the top and side bar layouts, the templates include an "Example" topic if the corresponding topic does not exist. Copy the example topic rather than changing the example directly.)

Top bar

Customizing the Top bar

Foswiki ships with WebTopBarExample. In order to tailor the Top
bar, navigate to WebTopBarExample and use the More topic actions
link to copy it to WebTopBar. Note that although the name suggest
that this is "per web", the WebTopBar is a site-wide setting.

You can also set WEBHEADERART per web, by setting the preference in the Web's WebPreferences.

Note: For more precise control, the top bar can be controlled using CSS, however: If WEBHEADERART defaults, or is set to anything other than the empty string, the CSS for the #foswikiTopBarContents is expanded inline from templates/css.pattern.tmpl. Overrides using css files will not be possible.

I want to hide the side bar

Buttons

I want to hide the edit buttons from certain users

It may defy the wiki-ness of your Foswiki installation, but in certain circumstances it could be useful to hide the edit buttons from users that are not logged in, for instance for clients that have gotten viewing rights only.

Using either a template or a topic template, 'empty' top:toolbarbuttons and topicaction by writing:

Other page parts

I want to insert text outside of the topic content

PatternSkin has 2 'buckets' to write additional content to: contentheader and contentfooter, both defined in view.pattern.tmpl. These containers can contain text or html and are placed directly before and after the topic text.

Note: the contentheader and contentfooter templates are
PatternSkin specific names for the beforetext and aftertext templates. For best
compatibility with other skins, you should override beforetext, and/or aftertext.

To put contents before the main text, use the custom skin approach as described above.
So our custom template contains:

%TMPL:INCLUDE{"view"}%
%TMPL:DEF{"beforetext"}%This is the text before%TMPL:END%

Use the same procedure for contents to be put after the topic text:

%TMPL:INCLUDE{"view"}%
%TMPL:DEF{"aftertext"}%This is the text after%TMPL:END%

Hint: These templates are often used by
extensions to add to the page above or below the topic contents. Rather than
simply replacing them, consider extending them by using the TMPL:PREV option
to expand the previous definition of the template.